This invention relates to the marbleization of materials extruded from injection machines of the screw type.
Marbleization is often desired in molded parts for esthetic reasons. The marbleization is in the form of irregular streaks and striations, of varying intensity and color, in the finished article.
In the case of molded articles the desired marbleization has been achieved by using injection machines of the plunger type. Colorant pellets are added to the regular feed pellets at the feed hopper of the machine. After being melted, the colorant and feed materials are driven by a plunger into an associated mold. Since there is little or no mixing in a plunger machine, the colorant and feed materials tend to remain separate. This separation is carried into the mold and the desired marbleization is easily achieved.
By contrast, in an injection machine of the screw type, the addition of colorant pellets to the regular feed pellets merely modifies the coloration of the output product, as illustrated by U.S. Pat. No. 3,854,630 which issued Dec. 17, 1974. The reason is that there is invariable mixing in screw machines and the output melt is relatively homogeneous.
As a result, successful commercial marbleization of molded products has been achieved only with plunger machines. The latter, however, do not produce uniform marbleization and cannot be used with unbalanced molds. In addition, they are slower and less efficient than screw machines. As a result, a number of attempts have been made to produce marbleized products with screw machines.
One such attempt was disclosed in U.S. Pat. No. 3,817,675 which issued June 18, 1974. In this patent a colorant hopper is positioned to apply feed pellets at the melt interface between a special "adiabatic" screw and a mixer. It is apparent that the provision of mixing stages beyond the place of introduction of colorant material tends to mix the colorant with the base material. Moreover, although it is contended that a mottled or marbleized effect can be achieved, it is acknowledged that this requires minimization of the number of mixing stages.
Attempts also have been made to achieve the desired marbleization by injecting colorants into the nozzle cavities of screw machines. The nozzle pressures are so great that injection of the colorant is difficult. In addition, when the colorant does manage to enter the nozzle cavity it produces a void that causes undesirable delamination in the molded parts.
Accordingly, it is an object of the methods and apparatus hereafter disclosed to enhance the efficiency and rate of marbleization of molded parts. A related object is to achieve marbleization with injection machines of the screw type.
Another object is to eliminate the need for reliance on plunger machines and special screw machines to achieve marbleization. A related object is to avoid the requirement for using an adiabatic screw. Another related object is to achieve marbleization with standard screw injection machines.
Still another object is to achieve marbleization in both balanced and unbalanced molds. A further object is to achieve relatively uniform marbleization in both balanced and unbalanced molds.